1. Technical Field
The present invention is generally directed to electromagnetic refrigeration systems. More specifically, the present invention is directed to apparatus and methods for performing switching of heat flow in electromagnetic refrigeration systems.
2. Description of Related Art
Recent developments in magnetocaloric materials have suggested that magnetic refrigerators with high thermodynamic efficiency may be practical at room temperature. Magnetic refrigeration is based on the magnetocaloric effect, i.e. the ability of some materials to heat up when magnetized and cool when removed from the magnetic field. The use of magnetocaloric materials provides a relatively safe and environmentally friendly alternative to the chlorofluorocarbons and hydrochlorofluorocarbons used in traditional vapor-cycle cooling systems.
FIG. 1 is an exemplary diagram illustrating the magnetic refrigeration cycles of an adiabatic magnetic refrigerator system. As shown in FIG. 1, refrigeration starts, in a first phase of the cycle, with the placement of a magnetocaloric material 110 within the magnetic field of a magnet 120. As the magnetocaloric material 110 enters the magnetic field, the electron spin of the atoms of the magnetocaloric material 110 align thereby causing the material to heat up.
In a second phase of the cycle, a cool fluid 130 is pumped into the system to absorb the heat from the magnetocaloric material 110. Thus, cool fluid 130 is pumped into the system and hot fluid 140 is pumped out of the system. In this way, heat is rejected from the magnetocaloric material 110 due to cooling from the cool fluid 130.
In a third phase of the cycle, the magnetocaloric material 110 is removed from the magnetic field. This causes the electron spin of the atoms of the magnetocaloric material 110 to randomize which increases entropy and draws heat from the lattice of the magnetocaloric material 110. By drawing heat from the lattice of the magnetocaloric material 110 in an adiabatic setting, the magnetocaloric material 110 cools.
In the last phase of the cycle, fluid 150 is pumped into the system. Heat from the fluid 150 is absorbed by the atoms of the magnetocaloric material 110 thereby reducing the temperature of the fluid 150. As a result, cold fluid 160 is pumped out of the system.
Systems for implementing the magnetic refrigeration cycle described above are mechanically complex. For example, in xe2x80x9cMagnetic Refrigerator Gets Down and Homey,xe2x80x9d Science News Online, Vol. 161, No. 1, Jan. 5, 2002, which is hereby incorporated by reference, a magnetic refrigeration system using a rotating ring is described.
In this system, the permanent magnets are stationary and a magnetic ring use used to move a gadolinium powder, stuffed into pockets of the ring, in and out of magnetic fields generated by the permanent magnets. A fluid is pumped into and out of the system to carry heat away and to provide a cooling fluid for refrigeration. The system requires a complex mechanical drive for rotating the ring, pumping the heat-conducting fluids in and out of the system, complex fluid sealing, and synchronizing the movements of the heat-conducting fluids through portions of the rotating ring.
Thus, as described above, these known magnetic refrigerator systems need moving parts and/or advanced fluid flow to absorb or reject heat from the magnetocaloric material. In principle, similar constraints bound the possibility of using electrocaloric (pyroelectric) materials. Furthermore, since magnetocaloric materials undergo very small temperature changes with each magnetic field cycle, e.g., 3 degrees Kelvin per Tesla of magnetic field, around room temperature systems need to be cascaded to realize refrigerators which provide cooling at useful temperature differentials, e.g., 20-30 degrees Kelvin. The complexity of fluidic schemes and the moving parts reduce the efficiency of cascaded coolers and reduce the reliability significantly. Moving parts and advanced fluid flow systems increase the cost of magnetic refrigerators and provide additional sources of failure.
Thus, it would be beneficial to reduce or eliminate moving parts and fluid flows in magnetic refrigerator systems in order to provide simpler and more reliable mechanisms that can be more readily cascaded or paralleled to provide sufficient cooling and temperature differential for practical use. Moreover, it would be beneficial to provide miniature and solid state mechanisms for controlling the switching of the heat flow to and from the magnetocaloric material in magnetic refrigeration systems.
The present invention provides an apparatus and methods for performing switching of heat flow in magnetic refrigeration systems. In one embodiment of the present invention microelectromechanical (MEM) switches are provided for switching from a heat absorption phase and a heat rejection phase of a magnetic refrigeration cycle. With this embodiment, when a magnetocaloric material is subjected to a magnetic field, the magnetocaloric material generates heat. At this time, a first MEM switch between a heat source (cold end) and the magnetocaloric material is placed in an xe2x80x9coffxe2x80x9d state. A second MEM switch between the magnetocaloric material and a heat sink (hot end) is placed in an xe2x80x9conxe2x80x9d state so that heat from the magnetocaloric material flows to the heat sink.
When the magnetic field is removed, the magnetocaloric material cools. The first MEM switch is placed in an xe2x80x9conxe2x80x9d state and the second MEM switch is placed in an xe2x80x9coffxe2x80x9d state. Thus, heat flows, or is absorbed, from the heat source to the magnetocaloric material.
Similar operation is performed in other embodiments of the present invention in which the MEM switches are replaced by thermoelectric switches. The thermoelectric switches operate such that an xe2x80x9conxe2x80x9d state is defined as the state enabling heat flow by virtue of the thermal conductivity through the thermoelectric switch. An xe2x80x9coffxe2x80x9d state is defined as a net zero heat flow through the thermoelectric switch. The xe2x80x9coffxe2x80x9d state is accomplished by providing a thermoelectric cooling effect current that is just sufficient to offset the heat flow through the thermoelectric switch due to the thermal conductivity of the thermoelectric switch.
Various arrangements of the thermoelectric switches are possible. In some embodiments, the thermoelectric switches are xe2x80x9cdirectly coupledxe2x80x9d thermoelectric switches, meaning that they are energized by a direct electrical coupling to a current source. In other embodiments, one or more of the thermoelectric switches are xe2x80x9cinductively coupledxe2x80x9d thermoelectric switches, meaning that they are energized indirectly by a magnetic coupling.
The magnetic refrigeration system of the present invention may be cascaded or paralleled to provide greater cooling temperature or capacity. In one embodiment, the cascading involves a plurality of magnetic refrigeration systems staged such that they each share at least one thermal sink/source with another magnetic refrigeration system. In another embodiment, the magnetic refrigeration systems may be staged such that the magnetocaloric material of a next stage acts as a hot end of the previous stage. The cascaded magnetic refrigeration systems may be arranged in a stacked manner, parallel manner, or the like.
These and other features and advantages of the present invention will be described in, or will become apparent to those of ordinary skill in the art in view of, the following detailed description of the preferred embodiments.